The use of composites is becoming more common in the transportation and other industries. For example, composites are used in the construction of aircraft fuselages, boat hulls and automobile bodies. Because of this, much more emphasis is being placed on the re-work or repair of these high value composite structures and the technologies used to improve the first pass quality of the repair to avoid the time and cost of re-working the repair.
Because the repair of composite structures involves the heat curing of composite plies and adhesive applied to a repair area of the composite structure, the thermal management of the composite plies and the adhesive is a critical part of the repair. Composite plies and adhesives have a specific cure temperature at which they can be successfully cured. This cure temperature will vary with the materials being used to execute a repair. The process instruction or manufacturer data sheet will specify the required cure temperature and dwell time at this temperature. Additionally, they will specify maximum allowable deviation from this set point. The allowable deviation can be as small as ±5° F. Any temperature excursions beyond the allowable range can result in a rejected repair. To further complicate matters, composite structures can vary in thickness considerably across the repair location. The thick areas of structure typically run much cooler than the thin areas. If the repair mechanic does not correctly manage the temperature gradient across the repair an unacceptable repair will result.
The current practice of thermal management of a composite repair first involves the repair operator applying the appropriately sized composite ply(s) and adhesive to the repair area of the composite structure. The repair operator then attaches thermocouples to the composite structure around and over the repair area using adhesive tape. Each thermocouple location is then labeled by the repair operator for identification purposes. The repair operator also individually labels the plug end of each thermocouple wire.
Some potential thermal management problems encountered in the initial steps of the repair include mislabeling thermocouples. It is not unusual for the labels given to each thermocouple location to be illegible. Additionally, the labels may be incorrect. For example, two thermocouples could be labeled with the same identifying number or a thermocouple could be labeled with one number and the plug end of the thermocouple wire could be labeled with a different number.
The repair operator then creates a hand drawn sketch or map of the composite structure repair area. Each individual thermocouple location and the identification number of the thermocouple are noted on the sketch.
However, the thermocouple sketch or map made by the repair operator could be inaccurate. The sketch of the repair area itself may be inaccurate or the scale or perspective of the repair area in the sketch could be skewed. This leads to confusion regarding the actual thermocouple locations relative to the repair area and the composite structure.
The repair operator then attaches any required separator films, bleeders and caul sheets on the composite structure over the repair area as required for the particular repair procedure. These obscure the actual locations of the thermocouples relative to the repair area on the composite structure.
The repair operator, to the best of their ability, then attaches a heat blanket to the composite structure over the repair area without having a direct view of the actual repair area which is hidden under the separator films, bleeders and caul sheets that have previously been attached to the composite structure. Thus, when the heat blanket is placed over the repair area and the thermocouples, it may not be properly centered relative to the repair area and the thermocouples. If the heat blanket is not centered over the repair area, the heat provided by the heat blanket to the repair area and the thermocouples could be very non-uniform.
The operator then attaches any remaining separator films or bleeder layers to the composite structure over the heat blanket. A vacuum bag is then attached to the composite structure over the layers previously attached. The repair operator then attempts to locate the position of each thermocouple under the vacuum bag and mark and label on the vacuum bag each thermocouple location and the identification of the thermocouple. Sometimes a bump is visible at each thermocouple location, but in many cases, the caul sheets and vacuum bag totally obscure the thermocouple locations. In these instances the repair operator must rely on the thermocouple sketch or map to determine the locations of the thermocouples relative to the composite structure repair area. Often the determinations of thermocouple locations are best guesses.
The repair operator then plugs each plug end of each individual thermocouple into a hot bonder. Complicated repair areas having irregular shapes or large repair areas can require forty or more thermocouples to be used over the repair area, increasing the possibility for mistakes being made. Each thermocouple transmits temperature information of the thermocouple to the hot bonder which is then displayed on a screen of the hot bonder.
The repair operator then begins the cure cycle for the composite structure repair area. The hot bonder is activated and controls the heat blanket to apply heat to the repair area and the thermocouples positioned over the repair area. The operator monitors the temperatures of the thermocouples displayed on the hot bonder screen. If a particular thermocouple is seen to be too cold on the display of the hot bonder screen, the operator consults the hand-drawn sketch or map and the markings of the thermocouple positions and identities on the vacuum bag to determine the location of the particular cold thermocouple under the bag and relative to the repair area. The repair operator then places external insulation on the vacuum bag over the area surrounding the particular thermocouple that is reading too cold to increase the temperature of this area. The repair operator then returns to the hot bonder screen to monitor the thermal response of the thermocouple that was reading too cold on the hot bonder screen.
The external insulation applied to the vacuum bag during the curing process obscures the thermocouple markings placed on the vacuum bag by the repair operator. The repair operator is unable to see the markings identifying the thermocouple and its position relative to the repair area under the insulation. This can cause confusion of the repair operator, especially if a repair is turned over to another operator on a different shift, during the curing process resulting in inconsistencies in the quality of the composite repair.
The operator continues to monitor the hot bonder screen to identify any particular thermocouples reading too low that would require insulation placed on the vacuum bag over the particular thermocouples for the duration of the composite repair cure, which is typically 4-8 hours. During this time, the operator may add and remove external insulation from the repair many times, as needed.